This application claims priority to German Application No. 100.03.638.4 filed on Jan. 28, 2000 and International Application No. PCT/EP01/00859 filed on Jan. 26, 2001, the entire contents of which are incorporated herein by reference.
The invention relates to a compensation current sensor, in particular for the measurement of DC and AC currents, in which the magnetic field, generated in a magnet core by a primary winding through which flows the current to be measured, is compensated by the compensation current in a secondary winding, wherein, to control the compensation current, at least one magnetic field probe affected by the magnetic field records deviations from zero flux and supplies this measured value via an evaluation circuit to an amplifier arrangement to generate the compensation current while, at the output of the amplifier arrangement, the secondary winding is connected in series to a terminating resistor so that a voltage proportional to the current to be measured is present at the terminating resistor.
Such compensation current sensors are known, for example, from DE 44 23 429 A1, DE 295 06 883 U1, and DE 295 20 066 U1. Their functional principle is described there in detail.
The greater the current through the primary winding, said current to be measured, is, the greater the magnetic flux generated by this current is and the greater the section-wise magnetization of the magnet core which arises through the leakage flux. The leakage flux is the magnetic flux which penetrates the magnet core only partially. However, the proportionality between the current through the primary winding and the current through the secondary winding ends if the current is so great that the saturation magnetization of the magnet core is reached by the leakage flux in a section of the magnet core. Therefore, the measurement range of the current sensor is restricted by the saturation magnetization of the magnet core.
The greater the core cross-section of the magnet core is, the greater the current to be measured is through which the saturation magnetization of the magnet core is reached, that is, the greater the measurement range is. However, to reduce the production costs and the size of the current sensors, the smallest possible core cross-sections of the magnet core are strived for.
The objective of the invention is to improve compensation current sensors of the class stated initially with the aim of significantly expanding their measurement range without the magnet cores, and thus the outer dimensions, being noticeably increased.
The objective is realized by a compensation current sensor of the class stated initially which is characterized by the fact that means are provided which couple a part of the primary winding""s magnetic flux, which is not coupled into the magnet core, into the compensation apparatus formed by the secondary winding and magnetic field probe and simultaneously back-couple the magnetic leakage flux of the secondary winding into the compensation apparatus. Through the inclusion of these two magnetic fluxes in the measurement, the measurement range can be dramatically increased without increasing the structural volume of the compensation current sensor.
Typically as means, metallic shields are provided which encircle the magnet core, lie in a plane with the magnet core, and have an opening in the area of the compensation apparatus so that the part of the magnetic flux of the primary winding is coupled into the compensation apparatus formed by the secondary winding and magnetic field probe and simultaneously the magnetic leakage flux of the secondary winding is back-coupled into the compensation apparatus.
In the case of the present compensation current sensor, the magnet core has a first section around which at least a part of a secondary winding is wound. A magnetic field probe is disposed in the area of this first section. The secondary winding and the magnetic field probe form the actual compensation apparatus of the compensation current sensor.
A primary winding is magnetically coupled to the secondary winding via the magnet core, where a part of the magnetic flux carried by the primary winding does not couple into the magnet core.
In a typical development of the present invention the means have the structure of a metallic shield which is at a distance from the magnet core and lies in a plane in which the magnet core lies. This shield encircles the magnet core up to a lateral opening in the area of the compensation apparatus where the magnetic flux penetrating the shield, which is composed of the magnetic flux stemming from the primary winding and the leakage flux stemming from the secondary winding, is coupled into the magnetic field probe.
The shield described above prevents in addition an increased sensitivity to external magnetic fields. Surprisingly, the shield does not only cause a reduced sensitivity to interference magnetic fields but rather it clearly expands the measurement range by the above-described coupling of the magnetic flux. The insight underlying the invention is that the shield accepts a part of the leakage flux so that a second section of the magnet core unloaded thereby only goes into saturation at higher currents. The shield acts as an additional magnet core with an air gap which is defined by the distance between the magnet core and the shield.
The close coupling between the windings is partially eliminated once more by the lines to the windings. Furthermore, for reasons of construction, an optimal coupling between the windings is only possible to a restricted extent. Through the elimination of the close coupling the second section becomes magnetized and thus the characteristic curve becomes nonlinear since the second section is not disposed in the vicinity of the magnetic probe and therefore its magnetization is not necessarily compensated to zero. However, due to the shield the magnetization is only done at significantly higher currents than without a shield. A better shield against external magnetic fields can in fact be obtained if the opening of the shield is disposed in the area of the second section, since the magnetic probe is more effectively shielded. However, it has been shown that in this case the measurement range is sharply reduced. This is due to the fact that the magnetic flux which is generated by the primary winding is supplied to the second section amplified through the shield so that the saturation magnetization is even reached at very small currents.
The magnet core can be completely closed in itself or, for example, have a gap in the area of the magnetic field probe. The magnet core is bent in the plane in which the magnet core lies.
The magnet core can, for example, be a toroidal core. The magnet core can also, for example, consist of two band sections which are formed by multiple folding of the band, which is described, for example, in WO 98/52052.
The shield is at a distance from the magnet core so that the shield acts as a shield. The distance is, for example, several millimeters.
Preferably the primary winding is guided around the secondary winding. Therefore, in this case the primary winding and the secondary winding are disposed on the same leg of the magnet core so that a closer magnetic coupling of the two windings is obtained and a significantly smaller core cross-section of the magnet core is needed, which clearly reduces the production costs of the compensation current sensor. Preferably, the core cross-section of the magnet core is smaller in the second section than in the first section. Through the smaller core cross-section the production costs can be reduced. Due to the small core cross-section the magnet core can be implemented as a massive part without too strong eddy currents occurring. In the area of the magnetic field probe a greater core cross-section can be advantageous in order to prevent interference magnetic fields from penetrating into the magnetic field probe.
The shield can, for example, be U-shaped. In this case, the opening of the shield extends along the entire first section. Since, outside of the plane in which the magnet core lies, no shield is provided, the compensation current sensor has a particularly small size.
Alternatively, the opening can be smaller so that a part of the shield runs along the first section.
The shield can also be disposed partially outside of the plane in which the magnet core lies. For example, the shield has the form of an open cylinder with a base. The shield is therefore beaker-like. Also in this case the opening of the shield can extend along the entire first section. xe2x80x9cCylinderxe2x80x9d is the designation for a body which is bounded by two parallel planes and one surface which arises through parallel displacement of a line along a three-dimensional curve. The three-dimensional curve does not absolutely have to be a circle, that is, the cylinder does not absolutely have to be a circular cylinder.
The shield preferably consists of a highly permeable material. For example, the shield consists of nickel iron.
It is within the scope of the invention to provide more than one part of the secondary winding. The parts of the secondary winding are wound around sections of the magnet core at a distance from one another.
The magnetic field probe is preferably disposed within the secondary winding. However, it can also be disposed next to the secondary winding but in the vicinity of the first section of the magnet core. The magnetic field probe can, for example, be disposed between two parts of the secondary winding at a distance from one another.